A known displacement device of the general type under consideration is disclosed in DE 10 2005 060124 A1. With the aid of a motorized drive unit, a carriage bearing the fifth wheel coupling can be displaced along the longitudinal axis of the vehicle on two parallel guide rails, and can be locked in a prescribed position. When the vehicle is traveling at high Autobahn speeds without intense curves, it is contemplated that the fifth wheel coupling will be disposed maximally close to the tractor cab, wherewith the gap between the cab and the front of the semi-trailer will be held at its minimum length, and under this regime vortices will be minimized. According to the cited known patent application, the control device for the displacement device is connected to the vehicle control system, so that information about vehicle speed can be retrieved and thus in normal operation the gap between the tractor and the semi-trailer can be adjusted. However, the problem occurs that if the vehicle attempts to negotiate a relatively narrow radius curve, there is the risk that swinging of the semi-trailer suspended on the tractor will result in a corner of the semi-trailer striking against the tractor cab. This problem can also arise in a situation of emergency panic braking, i.e. sudden breaking without a compensating steering maneuver; in such a situation it would be desirable if a braking signal were supplied to the control device, wherewith the braking signal could be evaluated and could be converted into a very rapid backward displacement of the carriage, so as to lengthen the gap between the tractor and semi-trailer.
A different state of the art is disclosed in DE 10 2004 045662 A1, which proposes a displacement device wherein a position sensor determines the position of a carriage. The signal from the position sensor is sent to a control device which may cause the displacement device to be engaged, thereby moving the carriage rearward, when the gap between the tractor cab and the front of the semi-trailer becomes too short.
However, in practice it is found that the determination of a braking signal and a vehicle speed are suitable only for determining a panic braking maneuver without a subsequent compensating maneuver. Under these circumstances it is generally satisfactory for the gap between the tractor and semi-trailer to remain unchanged, even if the fifth wheel coupling is in a forward-most position. If panic braking is accompanied by a simultaneous compensating steering maneuver, frequently this will lead to a corner of the semi-trailer striking against the cab of the tractor; the known device will not detect such a hazard. In a situation of a sharp load shifting reaction of the semi-trailer or when additional equipment is disposed on the front side of the semi-trailer, the known means of determination of the gap via determination of the position of the carriage are imprecise, introducing an additional factor of uncertainty as whether the semi-trailer can swing freely without striking against the rear of the tractor.
DE 10 2007 031318 A1 describes a device for changing the distance between a tractor and a semi-trailer coupled to the tractor, wherewith the distance is automatically adjusted depending on the speed and/or depending on the angle between the longitudinal axis of the tractor and the longitudinal axis of the semi-trailer. It is proposed that the maximum permissible speed-dependent tractor/semi-trailer angle should depend upon the steering angle of the steerable wheels of the tractor, or correspondingly should depend upon the angle of the steering wheel in the tractor. In the case of panic braking, the determination of the steering angle and the calculation of the resulting tractor/semi-trailer angle is poorly suited to avoid a collision of a semi-trailer with the tractor cab. If the vehicle is traveling at high speed, and while it is negotiating a long curve it encounters an obstacle, under some circumstances the driver must change the steering to “straight ahead” steering angle 0°, even though at the same time the actual tractor/semi-trailer angle may be 20-30°. With the known device, the semi-trailer will be drawn toward the tractor cab, which will lead to a collision. With the known device a given steering angle may be associated with a number of tractor/semi-trailer angles, depending on how far the vehicle has advanced into the curve. If the driver has maintained a given steering angle over a relatively long time interval, any of a variety of tractor/semi-trailer angles may have established itself. Conversely, a given tractor/semi-trailer angle may correspond to a variety of steering angles. As a result of this indefinite relationship between the steering angle and the tractor/semi-trailer angle, the speed-dependent steering angle is not suitable as a signal for controlling the displacement of the coupling.